This invention relates to a signal transmission system of a key telephone system, and more particularly a signal transmission system for transmitting and receiving various control signals (i.e. control data) among a plurality of key telephone sets connected in parallel between a pair of signal lines.
A key telephone system generally comprises a plurality of key on telephone sets, and a control device which receives from the telephone sets such control signals as an office line selection signal, a local line selection signal, and a reservation signal for controlling interconnection between local key telephone sets as well as interconnection between the office lines and the key telephone sets and for controlling display of the state of the office lines on display devices provided for respective key telephone sets. The respective key telephone set and the control device are coupled through by a pair of talking lines and a pair of signal lines transmitting the control signals. The control signals are digitallized and transmitted between the control devices and the key telephone sets. One example of such control system is disclosed in Australian patent publication No. AU-B-60038/80, for example. In this system, the operating power is supplied to respective key telephone sets from the control device through exclusive power supply lines or the signal lines. Each control device comprises an arithmetic operation unit having a plurality of input/output ports and performs various controls described above.
In the key telephone system, the transmission and reception of the control signals have been made in a time division mode in which the control signals are transmitted or received in time slots assigned to respective key telephone sets or through an exclusive temporary memory circuit connected between the control devices and the key telephone sets.
According to the latter method, however, the time necessary for transmitting and receiving the control signals increases in proportion to the number of the key telephone sets, thus shortening the processing time for executing various control operations in the control device at each predetermined period. Moreover, as the number of the exclusive temporary memory circuits for transmitted and received signals increases in proportion to the number of the telephone sets the construction of the control device becomes complicated.
Furthermore, in the prior art arithmetic operation unit, the number of the input/output units was limited to 4 through 8 so that it has been desired to transmit and receive signals among as many possible number of the key telephone sets with a limited number of inlet/output ports.
In a certain case, the control signals are transmitted through a transformer, but where the control signals are pulse width modulated, surge voltages are formed at the building up and building down portions of the pulse width modulated control signals. Consequently, when the waveforms of the received pulse width modulated signals are shaped by comparing them with a DC reference level, pulse width modulated signals different from the original pulse width modulated signals would result, thus failing accurate demodulation.
Although the pulse transformer may be constituted by a pulse transformer, such pulse transformer is more expensive than the ordinary transformer so that where a plurality of key telephone sets are widely distributed, the cost of installation increases.
In the key telephone system, speech signal lines and the control signal lines are generally installed closely adjacent, and since when digitalized control signals flow through the signal lines, pulse shaped noises would be induced in the speech signal lines. For this reason, in the prior art system, the signal lines were constituted by twisted paired lines for the purpose of eliminating the pulse shaped induced noises or to limit the amplitude of the control signal to a low level.
However, when the signal lines are constituted by twisted paired lines, the cost of installation increases. Further, when the amplitude of the control signal is limited to a low level, it becomes difficult to discriminate the levels at the receiving end, thus making to circuit design difficult.